A solid oxide fuel cell (hereinafter also referred to as “SOFC”) is a fuel cell device including an oxygen ion conductive solid electrolyte as an electrolyte, and two electrodes that are mounted to opposite sides of the electrolyte. Fuel gas is supplied to one electrode, and oxidant gas (air, oxygen, or the like) is supplied to the other electrode, and a power generation reaction is caused at a relatively high temperature to thereby generate power.
Specifically, the SOFC generally includes a fuel cell assembly (fuel cell stack) having a plurality of tubular fuel cells each including a fuel electrode (anode) layer, an air electrode (cathode) layer, and a solid electrolyte layer held therebetween. The SOFC is actuated by fuel gas and oxidant gas (air, oxygen, or the like) flowing from one side to the other side of the fuel cells. Unreformed gas (city gas or the like) as source gas is supplied from outside the SOFC. The unreformed gas is introduced into a reformer containing a reforming catalyst and reformed into fuel gas rich with hydrogen. The fuel gas is supplied to the fuel cell assembly.
The SOFC is configured to perform, as a starting step for reforming the unreformed gas with the reformer, a plurality of steps that includes a partial oxidation reforming (PDX) reaction step, an auto thermal reforming (ATR) reaction step, and a steam reforming (SR) reaction step, and then move to a power generation step. The SOFC can sequentially perform these steps to heat the reformer, the fuel cell stack, or the like to an operation temperature.
In such an SOFC, a gas seal structure that isolates a fuel gas channel from an oxidant gas channel is generally provided in order to separate a fuel gas and an oxidant gas. Various types of these gas seal structures have been studied. For example, a seal material made of silver (Ag) or mainly composed of Ag is desirably used for a gas seal structure because of high compactness (gas impermeability). Ag is also desirably used because Ag degradation due to oxidation or the like practically does not occur at a normal operation temperature (about 500° C. to 700° C.) of the fuel cells in the SOFC. Such an Ag seal material can be formed by sintering a brazed material on an area on which the Ag seal material is to be placed, and Ag is not oxidized even at a sintering temperature (about 1000° C.) at the time.
However, for example, as shown in Non Patent Literature 1, pores will be generated wholly inside the Ag due to water (water vapor) which is generated by the reaction of hydrogen and oxygen that are diffused in the Ag, when the Ag, which is contacted by both reducing atmosphere and oxidizing atmosphere, is heated to about 500° C. The pores, which are mainly formed along a grain boundary, will cause cracks to occur and develop in the Ag due to contact between the pores and portions where water (water vapor) occurs. Thus, the Ag degrades over time while the Ag expands (in other words “the porosity with Ag expansion”), leading to leaks of fuel gas and oxidant gas. The two gases, which are mixed and reacted to produce water, are consumed ineffectually, resulting in remarkable deterioration of power generation efficiency.